RE: [PATCH 3/9] iov_iter: refactor rw_copy_check_uvector and import_iovec

David Laight
2020-09-23T14:38:38Z

From: Al Viro
> Sent: 23 September 2020 15:17
>
> On Wed, Sep 23, 2020 at 08:05:41AM +0200, Christoph Hellwig wrote:
>
> > +struct iovec *iovec_from_user(const struct iovec __user *uvec,
> > + unsigned long nr_segs, unsigned long fast_segs,
>
> Hmm... For fast_segs unsigned long had always been ridiculous
> (4G struct iovec on caller stack frame?), but that got me wondering about
> nr_segs and I wish I'd thought of that when introducing import_iovec().
>
> The thing is, import_iovec() takes unsigned int there. Which is fine
> (hell, the maximal value that can be accepted in 1024), except that
> we do pass unsigned long syscall argument to it in some places.

It will make diddly-squit difference.
The parameters end up in registers on most calling conventions.
Plausibly you get an extra 'REX' byte on x86 for the 64bit value.
What you want to avoid is explicit sign/zero extension and value
masking after arithmetic.

On x86-64 the 'horrid' type is actually 'signed int'.
It often needs sign extending to 64bits (eg when being
used as an array subscript).

David

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